Superconductors are materials which can conduct electricity with virtually no resistance when they are maintained at a certain temperature, referred to as the superconductive transition temperature (TC). For example, pure metals or alloys such as niobium-tin (Nb.sub.3 Sn) reach the superconductive state when they are cooled below 23 K. That degree of cooling requires the use of liquid helium, which condenses at 4 K. Liquid helium is expensive and is difficult to manipulate.
A major breakthrough in the commercial development of this technology cam in January of 1987, when a yttrium-barium copper oxide ceramic, reported to be Y.sub.1.2 Ba.sub.0.8 CuO.sub.4, was prepared which achieved superconductivity at a Tc of about 90 K. See M. K. Wu et al., Phys. Rev. Lett., 58, 908 (1987). This degree of cooling can be readily acomplished with liquid nitrogen (boiling point, bp, 77 K. or -196.degree. C.), which is much less expensive and easier to handle than is liquid helium (bp 4.2 K.).
The advent of liquid nitrogen-cooled superconductors could be a boon to utilities, industry, electronics, transportation and medicine. For example, power companies envision superconductive transmission lines, buried underground, that would carry current with no dissipative losses or generation of heat. Superconducting devices could lead to smaller, more powerful supercomputers. Because these chips produce no waste heat, they could be packed closer together, allowing the size of electronic boxes to be reduced. This size reduction means that signals would take less time to travel between switching devices and circuit elements, leading to smaller, faster computers.
Since the discovery of high temperature superconductivity in the 2-1-4 oxides (of the form La.sub.2-x.sup.Sr.sub.x CuO.sub.4), the 1-2-3 oxides (of the form YBa.sub.2 Cu.sub.3 O.sub.7-x), the 2-1-2-2 oxides of Bi and Tl (of the form Bi.sub.2 Ca.sub.1+x Sr.sub.2-x Cu.sub.2 O.sub.8+y or Tl.sub.2 Ca.sub.1+x Cu.sub.2 O.sub.8+y), and the 2-2-2-3 oxides of Bi and Tl (of the form Bi.sub.2 Ca.sub.2+x Sr.sub.2-x Cu.sub.3 O.sub.10+y or Tl.sub.2 Ca.sub.2+x Ba.sub.2-x Cu.sub.3 O.sub.10+y) it has been apparent that the incorporation of these materials into existing and new technologies will require the solution of a large number of materials-related problems. Analogous problems can be anticipated for all such CuO-based superconductors, including systems presently in the early stages of development.
First, there are issues related to materials synthesis so that structures can be fabricated with predetermined shapes, sizes and current-carrying ability. These range from macroscopic to microscopic. Second, there are challenges related to the fabrication of superconducting thin films on a variety of substrates, with Si being an obvious choice from the perspective of microelectronic devices. Third, there are issues related to the formation of stable ohmic contacts, particularly for small samples and thin films. Fourth, there are problems related to the passivation, protection or encapsulation of small structures such as fibers or thin films, so that the superconducting oxides can be used under a wide range of environments.
Early work has shown that the surfaces of these high-Tc ceramics are highly reactive. In particular, adatoms which are reactive with respect to oxide formation cause disruption of the 2-1-4, 1-2-3, and 2-1-2-2 surfaces by withdrawing oxygen from the lattice to form undesirable oxide overlayers with thicknesses that are probably kinetically limited. In a broad sense, passivation layers act to prevent molecular motion across the layer, providing a stabilizing environment which protects the surface of the superconductor from the ambient atmosphere. More specifically, the passivation layers can serve as barriers against loss of oxygen from the superconductor, can isolate one device from another, can act as components in the semiconductor structure, or can provide electrical isolation of multilevel conductive systems.
H. M. Meyer et al., in Appl. Phys. Lett., 51, 1118 (October 1987) reported that evaporated gold films can passivate La.sub.1.85 Sr.sub.0.15 CuO.sub.4 superconductor surfaces against chemical attack. In an attempt to coat super-thin films with dielectric films, Y. Ichikawa et al., in J. Appl. Phys., 27, L381 (1988) used rf-magnetron sputtering to deposit Nb.sub.2 O.sub.5 films and Al.sub.2 O.sub.3 films on films of the Y-Ba-Cu-O superconductive ceramic disclosed by M. K. Wu et al., cited hereinabove. The thickness of the dielectric films was reported to be from 600-1350.ANG.. However, these layers deleteriously modified the crystal structure of the Y-Ba-Cu-O films, leading to a broadened superconducting transition zone and a lowered Tc. This group also reported Ba atom diffusion into the dielectric layer, indicating substantial disruption of the ceramic lattice.
Therefore, a need exists for methods to apply stable, effective passivation layers to superconductive ceramic oxides which do not disrupt the useful properties of the superconductor.